According to new research, in the same 2013–2017 timeframe that China managed to reduce these ultra-fine atmospheric PM 2.5 particles (which measure less than 2.5 microns in diameter, making them dangerous to people’s lungs), another source of harmful air pollution has actually increased: ozone.

By using a new network of over 1,000 air quality monitoring stations across China, researchers at Harvard University and China’s Nanjing University of Information Science & Technology (NUIST) discovered ground-level ozone pollution is rising, especially in China’s large ‘megacity clusters’, such as Beijing and Shanghai.

As for why the ozone levels are on the rise, the researchers say it’s directly related to the corresponding reduction in PM 2.5 pollution, because those particles act like a sponge for chemicals called hydroperoxy radicals that promote ozone production.

Now that there’s less fine particle matter in China’s air to soak up those hydroperoxy radicals, however, ozone production is ramping up.

“There was so much particulate matter in Chinese cities that it stunted the ozone production,” says atmospheric chemist Daniel Jacob from Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS).

“We haven’t observed this happening anywhere else because no other country has moved this quickly to reduce particulate matter emissions. It took China four years to do what took 30 years in the US.”